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Marine Battery System Hookups | Isolators | Selector Switches | Charger Systems  


How to Connect Isolators

The isolators are connected between the alternator(s) and the batteries. The difficulty in connecting isolators is due to the large variety of alternators/regulators. Although there are specific terminals (wires) involved in such connections, the alternators/regulators come in many different configurations and, furthermore, new configurations are introduced every year. This makes it difficult to generate diagrams that will cover every configuration.

The following attempts to identify the pertinent terminals (wires) of the alternator/regulator and to show how the isolator interfaces between these terminals and batteries.

  1. Pertinent Terminals/Wires


    1. DC Output This terminal supplies a positive DC voltage of approximately +14.5 V and it is used to charge the battery(s). The wires connected to it vary in color (red, orange, purple, etc.) but they are heavier gauged than the wires connected to the other terminals.
    2. AC Output There are two AC terminals (wires). They supply a square wave voltage with approximately 50% duty cycle and amplitude of +14.5V (i.e. Half the time the output is +14.5 V and half the time is 0 Volts). Because the voltage is only half the time on, a DC voltmeter will usually read approximately +7 volts. One of the two outputs is often used as the input to the tachometer. The two AC output wires are of the same color (yellow, green, etc.) and not connected to the same point of the terminal strip.
    3. Sense This terminal (wire) is used to protect the batteries from being overcharged. In essence this terminal is connected to the positive part of the battery and monitors its voltage value. When the battery is fully charged, the regulator turns off. When the battery voltage drains down, the regulator turns on and the battery is charged again. Usually the wire connected to the sense terminal is the same color as the DC output wire but of lighter gauge. Not all alternators/regulators have a dedicated sense terminal (wire). Some use the DC output terminal to perform the sense function.

    In many cases there are more Terminals/Wires from or to the alternator/regulator. They are not pertinent to isolator interface and should not be disturbed.


      In general there are two major alternator/regulator configurations.
    • Alternators with external regulators, found mostly on outboard engines and
    • Alternators with internal regulators, found mostly on inboard and 1/0 engines.


  2. Wiring isolators to engines with external regulators and dedicated sense wire

    The isolator is connected between the DC output terminal of the regulator and batteries as shown in the diagram.

    Schematic I1

    The sense wire is connected directly to the battery and it should not be disturbed. To interface the isolator, disconnect the DC output from the terminal strip of the regulator and connect it to the B1 terminal of the isolator.

    Using #8 gauge wire connect the A terminal of the isolator to the recently vacated DC output location on the terminal strip of the regulator.

    Using #8 gauge wire connect terminal B2 of the isolator to the positive post of the second battery.

    NOTE: On some regulators, one or more wires of the same color are connected to the sense wire. KEEP ALL THESE WIRES TOGETHER.


  3. Wiring isolators to engines with external regulator and without dedicated sense wire

    This type of regulators uses the DC output to sense the status of the battery. The isolator cannot be interfaced between the DC output and battery because the sense function will be disabled.

    In this case the charging voltage through the isolator is tapped at one of the two AC Terminals of the regulator.

    Schematic I2

    If there is a third battery it can be connected to the B1 terminal of the isolator as shown below.

    Schematic I3


  4. Wiring isolators to alternators with internal regulator and dedicated Sense terminal

    This configuration is similar to #2. The sense wire should not be disconnected and the isolator is inserted between the DC output of the alternator and batteries as shown below.

    Schematic I4


  5. Wiring isolators to alternators with internal regulators and without dedicated sense terminal

    This configuration is similar to #3. The DC output wire should not be disconnected and the charging voltage is taken from one of the AC terminals, as shown below:

    Schematic I5


  6. Twin Engine Configuration

    For this configuration an isolator with two A terminals is required. Each A terminal is connected to one alternator/regulator in the same way described previously. In the following example the alternator has a built in regulator and dedicates sense wire.

    Schematic I6


  7. Shortcomings

    When one alternator/regulator charges more than one battery but Senses the status of only one battery there are two shortcomings. Assume that the regulator senses BATT 1 and charges BATT 1 and BATT 2.

    1. If BATT 1 is not fully charged, the regulator will supply charging energy to both batteries thus BATT 2 is being charged whether it needs it or not. This situation may cause overcharging of BATT 2.
    2. If BATT 1 is fully charged the regulator will not supply charging power to either battery. Thus BATT 2 will not be charged even if it may need to. This situation is very probable when BATT 2 is used for the Accessories and BATT 1 is used to start the engine. During cruising both batteries get charged but only one battery (BATT 2) is being drained by the accessories. As soon as BATT 1 is fully charged the regulator turns off the charging power, and BATT 2 continues to drain out.


  8. Solution

    The solution to the above shortcoming is to be able to switch the sensing terminal from one battery to another. The most appropriate switch to be used is the Smart Switch Guest Model #2200. The Smart Switch has a built in digital voltmeter so the status of each battery can be monitored. Two wiring configurations (alternators with and without dedicated sense wire) are shown below.

    Schematic I7
    Alternator with dedicated Sense Terminals

    Schematic I8
    Alternators without dedicated Sense Terminals

    During cruising, the status of each battery is checked, and the switch is left in the position where the battery is lower.



Alternators with EXTERNAL REGULATOR:

Remove regulator field wire (regulator "F" terminal) and connect to new wire leading to Guest battery switch field terminal (either terminal).

Add another wire from the other switch field terminal and connect to regulator "F" terminal.

Use #14 AWG wire for new circuit.

Carefully tape connections for proper insulation.


If it is easier to reach the alternator, remove alternator field wire (alternator "F" terminal) and connect to new wire leading to Guest battery switch field terminal (either terminal).

Add another wire from other switch terminal and install to alternator "F" terminal.

Use #14 AWG wire for new circuit.

Carefully tape connections for proper insulation.

Alternators with INTERNAL REGULATOR:

The field circuit wire on alternators with internal regulators is located within the alternator housing and it is impractical to use the field disconnect feature. Protection of the alternator diodes can be achieved by installing a Guest Isolator #2401 (for 70 ampere alternators) or other appropriate model.

The function of #2100, #210, #2300A, #3100, #3200 selector switches will not be affected if the field circuit disconnect feature is not used.

Schematic 3

Schematic 4